The parent application identified above describes a method and apparatus for performing twin overedging and seaming in one high speed operation, and in particular to an improved method and apparatus for automatically aligning the fabric plies prior to such a sewing operation. The apparatus of this application senses the transverse position of corresponding lateral edges of each of the two plies at a point in the path of travel just prior to reaching the sewing head and provides a friction wheel at a point in the path of travel of each ply prior to the sensing point. The angular position of each of the friction wheels is independently controlled to maintain the desired alignment of the fabric plies.
While the apparatus of the parent application operates satisfactorily in most applications, the apparatus does have certain limitations. First, there are two types of alignment errors which may occur for a given ply of material. The first is an offset error where the lateral edge of the ply remains parallel to the desired path of travel but is displaced a fixed distance from the desired path. The second type of error is an angular error where the lateral edge of the ply is at an angle to the path of travel. The single sensor located between the friction wheel and the sewing head can detect and generate corrections for either of these types of error. However, it is possible that if both an offset and an angular error exist, they will substantially compensate each other at the sensing point, resulting in an undetected and uncorrected alignment error at the sewing head.
Similarly, where large alignment errors exist, or where the fabric to be sewn has a curved lateral edge, as for example would be the case where a back pocket is being sewn on a pair of jeans, the single sensor between the sewing head and the friction wheel cannot detect changes soon enough or react quickly enough to variations to maintain the desired alignment, particularly when the sewing machine is being operated at a high speed, for example, 4000 to 5000 rpm.
Another potential problem with the existing apparatus when it attempts to stitch a curved seam or to overedge a curved edge is that the sensor in the path of travel prior to the sewing head, which sensor is aligned with the sewing head, will detect an error because the curved fabric is not aligned with the sewing head at the point in the path of travel where the sensor is located. The system, in attempting to compensate for this falsely perceived error, causes an alignment error by forcing the fabric into alignment at the sensing point, and thus out of alignment at the sewing head. Thus, the current apparatus is not adapted for use with a fabric ply having a curved lateral edge, either convex or concave, which is to be overedged or where curved seams are to be sewn.
Another potential problem with the apparatus of the parent application is that, when the sewing machine is operating at high speed, centrifugal force might cause the friction wheel to bounce or lift from the fabric, preventing the friction wheel from reliably performing the alignment function. To prevent this effect, a means must be provided for maintaining the pressure on the fabric by the friction wheel regardless of the machine operating speed. High speed operation also reduces the time available for making alignment corrections and may therefore require faster responses by the guide wheel to detected errors and/or larger angular corrections by the guide wheel for a given alignment error. The existing application does not have a mechanism for taking into account the operating speed of the sewing machine in making angularity corrections.
It is also desirable that the pressure applied by the friction wheel to the fabric ply be adjustable, either manually or automatically, to compensate for differences in fabric thickness, to possibly stretch one of the fabric plies where the plies are of uneven length, and for other purposes. It is also desirable that the sensing element be maintained steady regardless of any possible movement or bounce by the friction wheel during operation. This requires that a means be provided for independently mounting the friction wheel and sensors.